1. Field of the Invention
The present invention relates to a method for making multiple layer metallurgy such as double metal, spin-on-glass multilevel metallurgy device structures which are substantially free of field inversion.
2. Description of the Prior Art
Multiple layer metallurgy such as double metal, spin-on-glass multilevel metallurgy is now used in high density integrated circuits for electrical connection to the device elements located in and on semiconductor substrates. Spin-on-glass is a very desirable material to be used in such methods and resulting structures to overcome the irregularity or substantially nonplanar surfaces of the first level of metallurgy. This irregular or nonplanar surface problem causes loss of resolution in the lithographic masking processing. The problem increases with higher levels of metallurgy.
These problems have been recognized in the prior art and attempts have been made to overcome these topographical problems principally in the one micron and above feature dimensions. These techniques can be generally grouped in categories of planarization either involving etchback or nonetchback techniques.
In the case of etchback processing, typically a coating is formed on top of the irregular surfaces by an means of spin-on-glass or thermoplastic deposition techniques which produces a planar surface. A plasma, reactive or chemical wet etching is used uniformly across the planar surface to remove the deposited layer and the tops of the irregular humps or the like to produce a planar surface at the desired depth.
In the early nonetchback processing, the usual technique was to heat the dielectric layer, which was typically glass until the flow of the glass reduced the irregularities.
More recently the nonetchback planarization using a spin-on-glass sandwich has found interest at the about one micron feature dimension integrated circuit processing. This technique is described in U.S. Pat. No. 5,003,062 to Daniel L. Yen and assigned to the same assignee of the present invention. In this patent a sandwich dielectric structure is formed and used in one micron or less processing of spin-on-glass sandwiched between two silicon oxide layers. The spin-on-glass layer functions as the means for planarizing the irregularities.
In the Yen U.S. Pat. No. 5,003,062, spin-on-glass that has been cured still remains in the final product. However, workers in the field such as A. Malazgirt et al U.S. Pat. No. 4,986,878 has found that spin-on-glass even after curing by conventional techniques causes reliability problems, such as the presence therein of mobile ions, e.g. sodium to the point that they use the spin-on-glass in an etchback process for planarization and then completely remove the remaining material. Then a conventional insulator is deposited.
In the paper entitled "FIELD INVERSION IN CMOS DOUBLE METAL CIRCUITS DUE TO CARBON BASED SOGS" by D. Pramanik et al, a further discussion of the reliability problems in the use of spin-on-glass (SOG) is considered. They conclude that the use of SOG must be heavily restricted to overcome field inversion, that is a positive charge build up between the SOG layer and the other dielectric layers. They state that SOG must be purely inorganic phosphosilicate glass, since organic based SOG emanates hydrogen. Further, they require that etchback be used to reduce the thickness of the SOG to the bare minimum in the final product. Still further, they require that the layer covering the SOG be formed of a material that does not liberate hydrogen, such as silicon dioxide. Normal processes for depositing silicon nitride or oxynitride do liberate hydrogen.
It is a principal object of the present invention to provide a method and resulting device which do not have the restrictions for the use of SOG that the prior art require.
It is a further object of the present invention to provide methods involving the formations of a dielectric barrier layer below the SOG composite layer which allows the use of organic based SOG materials and makes the use of any etchback of the SOG unnecessary.